1. Field of the Invention
The present invention relates to laser drive circuits, and more particularly to drive circuits to which a modulating signal may be applied.
2. Related Art
Before considering such circuits in detail, it will be helpful to examine the laser behaviour. FIG. 1 shows a graph of light output power against input current for a typical semiconductor laser diode. The curve marked A illustrates a typical characteristic at room temperature. It is characterised by a slope S and by a turn on current I.sub.t. This characteristic is however temperature-dependent, and typical graphs for higher and lower temperatures are shown in curves B and C respectively. It is apparent from an examination of these graphs that in order to drive the laser with a modulating signal it is necessary to provide some standing current to bring the laser into an operating region, and to vary this current in accordance with the modulating signal. Reference D indicates a typical range of drive current for operation on the curve A, in which it can be seen that the light output varies between substantially zero and some desired maximum. It can also be observed that applying this same range of current at the higher temperature (Graph B) results in a lower maximum power output, and also results in the laser being driven considerably below cut-off. This is particularly unsatisfactory since, once driven below cut-off, an increasing current to being the device back into the operating region introduces a delay which can degrade performance when attempting to modulate with a high bit-rate digital signals. On the other hand, applying the same range of currents to the low temperature case (Graph C), a much higher light output is obtained, but with a considerable minimum light output (this is referred to as a low extinction ratio); this however causes problems in demodulation.
One solution to this problem is to use a Peltier cooler with appropriate control circuitry to maintain the temperature of the device reasonably constant. This however results in an increase in expense.
Variations in turn-on threshold may be accommodated by the use of a known mean-power controller, a schematic diagram of which is shown in FIG. 2. Here a laser diode 1 is driven with a current I and produces a light output L=S.I Watts. The light output is sensed by a back facet monitor photodiode 2 which drives a current of K.L. amps into a load resistor 3 of resistance R. The mean light output is determined by a voltage reference source 4 producing a voltage V.sub.ref and the voltage developed across the load resistor 3 is compared by an integrated transconductance amplifier 5 with the voltage developed across the load resistor 3 to control the current fed to the diode. A modulating current is fed to the laser diode 1 from an external current source connected at an input 6.
If the amplifier 5 has a transconductance-bandwidth product G, then the laser output ignoring any modulation input is: ##EQU1##
We see that the light output for .omega.=0 is independent of S, and thus the mean power setting is held constant. In the event that a modulating current I.sub.data is applied to the input 6, the light output is then given by: ##EQU2##
We see that here at high frequencies the gain is dependent on S, and thus the situation shown in FIG. 3 obtains, where the operating regions for the same current drive swing are shown. At high temperatures a poor extinction ratio is obtained, whereas at low temperatures the laser can be biased below cut-off, or even reversed biased, with the turn-on delay penalty. It can moreover be seen that the gain for low frequency data is low, falling to zero at d.c., the feedback control of the amplifier 5 effectively removing the d.c. component from the data. Thus this type of drive is suitable only for data having a symmetrical waveform; specifically it is extremely unsuitable for burst data drives such as may be used in TDMA systems such as passive optical networks.
According to the present invention there is provided a laser driver comprising a data input for receiving data signals; means for providing a feedback signal representative of the laser light output; a first amplifier having gain at d.c. and lower frequencies connected to receive the data signals, a d.c. reference signal and the feedback signal to provide current to the laser; and a second amplifier having gain at higher frequencies connected to receive the data signals the feedback signal to provide current to the laser.
Preferably the first amplifier is an integrating amplifier having, below a threshold frequency, a higher gain than the second amplifier and, above the threshold frequency, a lower gain than the second amplifier.
If desired the driver may have gain adjustment means whereby the gain provided by the driver to data signals may be rendered equal at d.c. and at a frequency above the passband of the first amplifier.